1. Field of the Invention
This invention relates to aircrew breathing gas regulators and is more particularly concerned with a regulator for use with a liquid filled aircrew G-protection suit.
2. Description of the Prior Art
The enhanced agility of modern high performance aircraft designs give such aircraft the ability to perform highly accelerative maneuver both at low altitudes and at high altitudes, e.g. in excess of 12000 meters (40,000 ft). To take advantage of this agility an aircrew member flying the aircraft must be protected against G-induced loss of consciousness (G-loc), as well as the effect of exposure to high altitude in the event of loss of cabin pressure.
The partial pressure of oxygen in air decreases with increasing altitude (decreasing total pressure) so that the concentration of oxygen in breathing gas supplied to the aircraft aircrew member must be increased with increasing cabin altitude to maintain the oxygen partial pressure above the minimum value necessary for it to be able to diffuse through the lung tissue and pass to the hemoglobin or red corpuscles in the blood. If, at aircraft operating altitudes above 12000 meters, there is total or partial loss of cabin pressure then the overall pressure of the breathing gas delivered to the aircrew member must be increased to a value above cabin ambient pressure so that the minimum critical oxygen pressure is maintained in the lungs, this being referred to as positive pressure breathing (PPB).
Positive pressure breathing at high altitude is aided by exerting pressure around the chest to assist the aircrew member in exhaling used gas from his lungs against the positive pressure in his breathing mask. To meet this requirement the aircrew member wears an inflatable counter-pressure garment ("jerkin") around his chest and back area which is inflated to the same pressure as the pressure in the breathing mask during positive pressure breathing, conveniently by being connected for inflation by breathing gas delivered to the breathing mask.
To counter the effects of high G-load it is usual at the present time for an aircrew member to wear an inflatable G-protection trouser garment which is inflated from a source of high pressure gas, such as engine bleed air. Inflation of the trouser garment may be in response to signals from one or more accelerometers located in the aircraft for sensing accelerative forces, or in response to movement of an inertia mass provided as part of an inflation control valve assembly. When inflated, the trouser garment restricts the flow of blood into the lower extremities of the body where it tends to be forced under the action of the G-load to which the aircrew member is subjected.
It has been found that protection against G-loc is further enhanced by providing positive pressure breathing during periods when high G-loads are being experienced and the G-protection garment is being pressurised. The increase in breathing pressure causes an approximately equal increase in heart level blood pressure thereby increasing the flow of blood to the brain.
Disadvantages of this arrangement are that the aircrew member must don two separate protection garments before commencement of an operation and each garment must have provision for connection to a source of inflation gas.
It is suggested now that protection of an aircrew member might be enhanced by a one piece liquid filled suit extending from ankle to neck. A suitable liquid would be one such as water, having a density approximating with that of blood. When such a liquid filled suit is being worn by an aircrew member sitting in his aircraft on the ground, the suit hydrostatic pressure will increase progressively from the vertically lowest region of the suit, thus being highest in the ankle region where the head of liquid is a maximum. In operation, when the aircraft performs a maneuver giving rise to acceleration along the vertical (G.sub.z) axis of the aircraft, the resulting G-force will cause the suit hydrostatic pressure to increase and, the hydrostatic pressure at any position within the suit will be given by the formula: EQU p=(Ng).rho.h
where:
p=hydrostatic pressure PA1 N=a number PA1 g=gravitational acceleration PA1 .rho.=density of liquid in the suit PA1 h=head of liquid at position within suit where hydrostatic pressure is to be calculated
In the case of positive accelerations, the increase in hydrostatic pressure in the lower regions of the suit will be greater than the increase in hydrostatic pressure in the upper regions of the suit and this difference will act to restrict the flow of blood into the lower extremities of the body. At the same time the increase in hydrostatic pressure in the chest region of the suit will tend to balance the higher pressure in the lungs of the aircrew member if breathing gas delivered by a breathing gas demand regulator is supplied at a pressure which is scheduled with respect to G (positive pressure breathing with G).
When an air inflatable G-protection garment is being worn, the breathing gas demand regulator may be arranged to be responsive to a pneumatic signal representative of pressure in the G-protection garment in delivering breathing gas for positive pressure breathing. However, such a pneumatic signal is not readily available from a liquid filled suit and there exists a requirement for a breathing gas demand regulator suitable for use with such a suit.
In meeting this requirement care must be taken not to detract from satisfactory operation of the liquid filled G-suit such as might happen if liquid were to leak from the suit. Also, operation of a breathing regulator must not be impaired by leakage of liquid from the suit into the regulator. Electrically operated sensors could be used to sense hydrostatic pressure in the suit but these are both expensive and unreliable.
A liquid filled G-suit in combination with a breathing gas regulator is disclosed in WO-A-9103278 (McDonnell Douglas) published 21 Mar. 1991, after the earliest priority date of the present application. In this disclosure hydrostatic pressure in the G-suit is sensed by a diaphragm which is connected by a mechanical linkage to a valve member regulating flow of breathing gas. A pressure differential across the diaphragm due to breathing demands of an aircrew member wearing the G-suit effects movement of the diaphragm and with it the linkage to cause the valve member to open. A disadvantage of this arrangement is that provision must be made in a supply line from the regulator to a breathing mask worn by the aircrew member, to prevent liquid from the G-suit flowing to the breathing mask in the event of rupture of the diaphragm. The arrangement disclosed in WO-A-9103278 for this purpose increases the breathing effort required of the aircrew member in making demands for breathing gas and, also, introduces an added weight penalty.
An embodiment (FIG. 27) of WO-A-9103278 designed to provide for increasing the pressure of breathing gas supplied to the aircrew member in the event of exposure to high altitude (i.e. low cabin pressure), has a mechnanism including an aneroid capsule which expands on exposure to high altitude to off-load a spring biasing a valve towards closure against the action of hydrostatic pressure of the diaphragm. This arrangement does not meet a current requirement that the higher of breathing gas pressures be provided for protection in the event of simultaneous exposure to G-load and high altitude because the effect of expansion of the aneroid capsule is additive to that of hydrostatic pressure acting through the diaphragm.
It is an object of the present invention to provide an aircraft aircrew breathing gas regulator suitable for use with a liquid filled G-protection suit and which meets the requirement for positive pressure breathing in protecting an aircrew member against G-load and/or exposure to high altitude (low cabin pressure).